1.Remove the dead pc power supply
Once you've disconnected all the leads, look at the rear of the case and find the three or four screws holding the power supply in place. Remove them, then gently lift out the PS, making sure that the cabling doesn't get tangled on, say, a RAM module or expansion card and accidentally pull it loose.

2. Install the new Pc power supply
Out with the old, in with the new. Make sure the replacement Pc power supply is set to the correct voltage, which for U.S. users means 115 (look for the little red switch on the back: If it's showing 230, slide it to 115).
Once the old PS is out, put the new one in its place and screw it in. Now just reverse the steps you performed: connect the leads to their respective outlets, replace any extracted components, and consult your photos to make sure you've left no lead unplugged. (You'd be surprised how easy it is to forget, say, the DVD lead, especially when you've spent 10 minutes trying to wedge in that pesky front-panel lead.)

3. Fire it up
Before you close up the case, make sure everything works. Connect the power cord to the Pc power supply, plug in your monitor and keyboard, and fire up the machine. Unless you've failed to connect the main leads to the motherboard, it should come right to life, just like before. Let it boot completely, then check to make sure there's power to the optical drive(s) and, if applicable, the front panel.